<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <div id="app">
        <input v-model="fullname">
    </div>
</body>
<script src="./js/lib/vue.js"></script>
<script>
    const vm = new Vue({
        el: `#app`,
        data: {
            fn: `tt`,
            ln: `ss`
        },
        computed: {
            fullname: {
                set(val) {
                    this.fn = val;
                    return `${this.fn}-${this.ln}`;
                },
                get() {
                    return `${this.fn}-${this.ln}`;
                }
            }
        }
    })
</script>

</html>